The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission and pertains generally to self-powered neutron detectors and more particularly to such detectors that have internally designed gamma compensation.
A variety of self-powered detectors presently exist for the measurement of reactor thermal neutrons. The large thermal neutron cross sections result in easily measured signal currents with minor contributions from gamma induced currents. As attention focused on an application of self powered techniques to the measurement of fast neutrons, reduced neutron cross sections resulted in greater relative contributions of the interfering gamma induced currents. Accordingly, in order to provide a representative fast neutron detector output, a method or technique is required which will effectively eliminate or significantly reduce the gamma current contributions.
Presently, in analogous state of the art devices, compensation for gamma induced currents is achieved by introducing a second detection device responsive only to the gamma effects. The neutron caused and gamma caused currents are then combined in a summing network to provide a measurement of the desired parameter (neutron representative current). This compensation technique necessitates the use of a two conductor shielded cable and generally requires a complex and difficult to fabricate structure.
One example of prior art devices can be found in the patent to Hilborn U.S. Pat. No. 3,375,370, which accomplishes gamma compensation by using two identical detectors, one of which is sensitive only to gammas. Currents generated by the two detectors are summed at the input to a differential current meter so that only the neutron induced current is indicated. A second example can be found in the patent to Linden U.S. Pat. No. 3,067,329, which provides gamma compensation in a manner similar to the Hilborn method except that the two detectors are constructed concentrically instead of separately. The Linden method requires two separate cables or a twin conductor cable coupled to current summing equipment to balance out the gamma induced current.
Preferably, a structural arrangement is desired for fast neutron, self-powered detectors, which enables direct, internal cancellation of gamma induced currents and eliminates the need for two coaxial cables or for twin conductor cables at the detector output. Desirably, measurements should be made with one single ended output current lead.